Optical measurement systems which use a laser triangulation sensor to obtain dimensional information about objects are known. For example, such systems may be arranged to determine the distance of objects from the sensor or the profile of objects within the field of view of the sensor.
Referring to FIG. 1, a conventional laser triangulation measurement device 1 comprises a light source 2 e.g. laser which is arranged to project a planar beam 4 (e.g. sheet) of light. The planar beam 4 is incident as a line 8 on an object or objects 5, 6 which lie in the field of view 7 of the device 1. Light reflected from the incident line 8 is collected by an imaging device 3, which may be a camera (e.g. having a charge coupled device (CCD) or an active pixel sensor (CMOS) device).
The images captured by the imaging device 3 are processed to determine a data representation of the physical geometry of the objects 5, 6. The processing may involved reference to a calibrated look up table or the like. Such processing is known.
FIG. 2 depicts two dimensions that may be determined using the data representation. The separation (gap G) or planar misalignment (mismatch or flush F) between adjacent surfaces may be determined, e.g. by performing suitable mathematical operations (e.g. line/radius fitting).
An example of a conventional optical triangulation sensor is the GapGun, manufactured by Third Dimension Software Limited.
It is often desirable to measure accurately the diameter (or radius) or edge profile of a hole formed in a measurement surface. For example, it may be particularly useful for measuring the edge profile of a countersink (i.e. a hole whose opening is tapered to receive a conical screw head or the like). To perform measurements of this type using an optical triangulation sensor it is desirable for the measurement axis (i.e. the line formed on the measurement surface when the planar light beam is projected thereon) to lie across the centre of the hole (i.e. on a diameter). For accurate measurements, the axis of the hole preferably lies in the plane of the light beam.
Often this is done by eye, e.g. relying on a user's judgement. This limits the accuracy and repeatability of measurements.
US 2010/0195116, which is incorporated herein by reference, disclosed an optical triangulation sensor having a guide element for centering the planar light beam across a hole. The guide element comprises a body receivable in the hole, the body having contact surfaces for abutting the hole circumference, the contact surfaces lying on a virtual conical surface whose axis is in the plane of the light beam. A void is provided in the body at each intersection between the virtual conical surface and the plane of the light beam to define a path for the planar light beam from the light source to the edge of the hole.
U.S. Pat. No. 7,542,135 discloses an alternative solution, in which the self centering device includes a conical cone-shaped member adapted to mate with the countersink, the conical cone-shaped member having a slot extending partially therethrough to allow the laser beam to pass across the countersink and hole.